Regeneration of photographic silver bleach solution



Patented July 18, 1950 REGENERATION OF PHOTOGRAPHIC SILVER BLEACH SOLUTION Eugene G. Seary, Irvington, N. Y., assignor to Pavelle Color Incorporated, New York, N. Y., a

corporation of Delaware No Drawing. Application May 4, 1946 Serial No. 667,485

2 Claims. (01. 95-88) The general object of the present invention is to provide a novel, practical and effective method of regenerating a photographic bleaching solution containing a soluble ferricyanide as its active agent.

In processing many photographic materials, the latter are subjected to the bleaching action of a solution of the type mentioned above. Usually, the solution also contains a soluble bromide which acts as an accelerator. In the bleaching action, the ferricyanide oxidizes the metallic silver in the photographic material to silver ion, and the bromide reacts with the silver ion to form silver bromide. The reaction of the ferricyanide with the silver is an oxidizing action and results in the reduction of the ferricyanide to ferrocyanide. In consequence, ferrocyanide ion accumulates in the bleaching solution bath as the normal bleaching operation proceeds with the result of slowing down the bleaching action. If the ferricyanide in the solution is potassium ferricyanide K3Fe(CN) e, the cyanide product furnished by the conversion is potassium ferrocyanide K4Fe(CN) s. The ferricyanide constituent of the bleaching solution may well be potassium ferricyanide, the less expensive di-potassium-sodium ferricyanide, or other alkali group ferricyanide. to ferricyanide, the bleaching solution may contain buffer salts such as sodium phosphates and water. The buffer salts are added for pH control. Volumetrically, water is the principal constituent of the solution. The ferricyanide is generally present in the solution in the ratio of 50 to 150 grams per liter.

The presence in the bleaching solution of a relatively small amount of ferrocyanide greatly reduces the bleaching capacity of the solution even though the unreduced ierricyanide component of the solution remains relatively high.

The purpose of the present invention is to reconvert the ferrocyanide constituent of a partially spent bleaching solution into ferricyanide. The need for a relatively inexpensive and practically feasible process of reconverting the ferrocyanide content of a partially spent bleaching solution into ferricyanide is recognized, and various methods of accomplishing the regeneration have been proposed. Thus, it has been suggested that the regeneration might be effected by electrolytic action, but that procedure is open to the practical objection that the apparatus required is expensive, requires close control, and the solution contains too much water for efiicient electrolytic cell action.

It has also been proposed to precipitate and remove ferrocyanide from the solution in the form of ferric ferrocyanide, commonly called Prussian blue, but that method seems impractical because of the difiiculty of filtering out the Prussian blue. Furthermore, the ferrocyanide is In addition I agitating the solution.

2 totally lost by this method and must be made up with fresh ferricyanide, which is economically undesirable.

It has also been proposed to regenerate the bleach solution by the use of bromine in some form involving appreciable dilution of the bleaching solution. In particular, it has been proposed to use sodium hypogromite in the form of an alkaline solution. The use of hypogromite as a regenerating agent is open to important practical objections. For one thing, sodium hypobromite is not a commercially available product and its preparation from bromine and sodium hydroxide is rather a dangerous process which must be conducted with extreme care. The dilution of the bleaching solution resulting from the addition of an adequate amount of sodium hypobromite is substantial and objectionable. Moreover, the addition of the hypobromite changes the pH of the bleaching solution. While the disadvantages, other than its dilution eiiect, attending the use of sodium hypobromite as a regenerating agent can be reduced and compensated for, more or less, by the addition of corrective chemicals, such corrective treatment requires time and close control, and is expensive.

I have found that it is practically feasible to regnerate the bleaching solution rapidly by adding bromine in gaseous, liquid or solid form, or by adding chlorine in gaseous form to the solution to be regenerated. In the form of the invention which I now consider preferable from the practical standpoint, I add liquid bromine to the solution to be regenerated While vigorously The liquid bromine added preferably contains no diluent, or, at least is so concentrated that its addition to the bleaching solution has no significant diluting efifect. The liquid bromine added to the partially spent, or exhausted, bleach bath, quickly oxidizes the accumulated ferrocyanide ion back to ferricyanide. The bromine also replaces the bromine ions which have passed out of the solution as silver bromide was formed. The addition of the liquid bromine in suitably concentrated form, dilutes the solution only to an infinitesimal extent, and does not change the pH of the solution. Furthermore, liquid bromine is a very much more efiicient regenerative agent than sodium hypobromite.

The bleaching solution may be effectively regenerated by bromine added to the solution in gaseous or solid form, as well as in liquid form. In the use of bromine gas in regenerating a bleaching solution, the latter may be scrubbed with bromine gas in various ways, as those skilled in the art will recognize. One eliective method consists in bubbling air through liquid bromine to form a gaseous mixture of air and bromine,

and then bubbling said mixture through the bromine is added in the form of a concentrated 10..

liquid.

When bromine; is used as the regenerative. agent, the regeneration of the bleaching solution takes little time and requires a minimum. of control. The bromine added to the solution should not be in. excess of the amount. required; to convert substantially all of the ferrocyanide. into ferricyanide. Because of the corrosiyeohars acter of bromine, free bromine in the solution would be seriously objectionable, not only be.-- cause it' would attack the solution containing; apparatus, but alsobecause it would injuriously attack the photographic material treated. However, I have found that it is not practically. dii-. ficult. to determine how much bromine shouldbe. v added to a partially spent solution to obtaina suitable regeneration effect. without risk of-leavingf free bromine in the solution. In general, approximately 0.2 gram of bromine should be added to the. partially. spent solution for. each. gram of ferrocyanide in the solution. The pres.- ence of free bromine in the-solution can be read-e ily detected by simple reaction color tests. when the regeneration of partially spent, bleaching solution is affected by withdrawing solution from: the bleaching chamber space for regeneration in: small batches without interrupting the bleaching action the injurious consequences of a small excess. of free bromine inthe regenerated solution will ordinarily be confined to the. regeneration apparatus. In such case, with the normal slow return to the bleaching chamber space of the regenerated solution, a small amount of free bromine therein will be immediately neutral-- ized in said space by the ferrocyam'de in the been withdrawn from the bleaching chamber.

space to a regeneration receptacle. Liquid bromine is addedto each batch undergoing 591131941 tion by allowing it to fiowfrom a bottlethroughv a glass tube having its discharge end immersed in the solution undergoing regeneration. That, solution is vigorously agitated during; the regene eration process. Such agitation of the bleaching solution may be effectedin any of the ways. customarily employed to continuously; agitate-thebleaching solution in the bleaching chamber,- space..- In practice, I have agitated thesolution; undergoing regeneration by means of apropeller; type mixer. It is apparent-that the regenerae. tion operation, may be a continuous one,,but-in-. the considerable practical use which I have :madee ofthe invention, the process has involved: successive treatment of batches of partiallyspentsoluetion withdrawnfrom the bleaching chambersandr returned to thelatter after its regeneration.

My experiments have shown that .foruse in re 70,

generating the above described bleachingrsolui-r. tion, chlorine gas is substantially equivalenttobromine gas. The low boiling; and solidifying; temperaturesof chlorine, minus. 33.6 C. and;

minus 102.6? 0., respectively, apparently preclude, 75.

4 the. use-of. chlorine in liquid or solid form as a practicalbleaching solution: regenerating material, though it is practically feasible to pass chlorine gas into the bleaching solution through a pipe including pressure reducing means directly from a container holding liquid chlorine under pressure. In general, approximately about .0875 of: .a gram of. chlorine should be added to the partially spent bleaching solution for each gram or ferrocyanide in the solution.

Having now described my invention, what I claim as'new and desire to secure by Letters Patent, is:

In the process of bleaching a silver image contained in a photographic emulsion layer which also. contains a dye. image, saidprocess-comprisingtreating the silver. image with a solution;

containing about. 50 to. grams per liter. of-

ferricyanide and a soluble halide whereby the is reduced to ierrocyanide, and then treating the.

partially spent bleaching solution which contains ferricyanide, ferrocyanideand-a halidewitlr a regenerating agent, the. improvement which consists. in adding, assaid regenerating. agent, asubstantially water free halogen selected from. the g-roup'consistingof bromine, and chlorine, to

the solution in substantially, the amount: required.

toconvert the ferrocyanide content of v the-solu tion into ferricyanide and to increase the-soluble halide inthe solution to substantially its original content withoutgiving the-regeneratedsolution a. free halogen content.

2: In the proces's of bleaching. asilver image contained in aphotograp'hic. emulsion-layer which also contains adyeimage, said process compris ing treating the silver imagewith asol-utioncon. taining about 50 to 15 9;grams per literrof ferri' cyanide and a soluble. halide whereby the silver in the silver image.:-is-oxidized to silver ions-whichcombine with the halide ions of the solution and. form a silver halide in the:emulsion and a relatively small amount: of the ferricyanide is reduced to. ferrocyanide, and then'treatingthe partially spent bleaching: solution which contains".

ferricyanide, ferrocyanide and a halideswith a regenerating agent, the improvement which consists, ,in-adding as .said regeneratingegent; bros mine. to the-solution insubstantially the amount requiredto; convert theaferrocyanide content. ofthe solution into ferricya-niderand:to;increase:the'

soluble halide in.:thezsolution.to substantially; its

original content, without: giving. the. regenerated:

solution-a gfreehalogen; content.

EUGENE G... SE-ARY;

I anrnnnnons orrnn The following references are off'recordllinthe.

file offtliis-patentz UNITED. S'ITATES .P-ATENTS Number:

Name-w Date- 1,286,890 Bullock; Dec. 3-} 191B": 1,589,041 Barsk-y! e June 15, 1926 2,261,672.. Barnes et -al. Novwl, 1941 2,346,077 Miller A'pr'. 4,1944 2,409,959 Ryanetal; Oct. 22,1946

OTHER REFERENCES London' pagefio particularlyicited. 

1. IN THE PROCESS OF BLEACHING A SILVER IMAGE CONTAINED IN A PHOTOGRAPHIC EMULSION LAYER WHICH ALSO CONTAINS A DYE IMAGE, SAID PROCESS COMPRISING TREATING THE SILVER IMAGE WITH A SOLUTION CONTAINING ABOUT 50 TO 150 GRAMS PER LITER OF FERRICYANIDE AND A SOLUBLE HALIDE WHEREBY THE SILVER IN THE SILVER IMAGE IS OXIDIZED TO SILVER IONS WHICH COMBINE WITH THE HALIDE IONS OF THE SOLUTION AND FORM A SILVER HALIDE IN THE EMULSION AND A RELATIVELY SMALL AMOUNT OF THE FERRICYANIDE IS REDUCED TO FERROCYANIDE, AND THEN TREATING THE PARTIALLY SPENT BLEACHING SOLUTION WHICH CONTAINS FERRICYANIDE, FERROCYANIDE AND A HALIDE WITH A REGENERATING AGENT, THE IMPROVEMENT WHICH CONSISTS IN ADDING, AS SAID REGENERATING AGENT, A SUBSTANTIALLY WATER FREE HALOGEN SELECTED FROM THE GROUP CONSISTING OF BROMINE AND CHLORINE, TO THE SOLUTION IN SUBSTANTIALLY THE AMOUNT REQUIRED TO CONVERT THE FERROCYANIDE CONTENT OF THE SOLUTION INTO FERRICYANIDE AND TO INCREASE THE SOLUABLE HALIDE IN THE SOLUTION TO SUBSTANTIALLY ITS ORIGINAL CONTENT WITHOUT GIVING THE REGENERATED SOLUTION A FREE HALOGEN CONTENT. 